Comment #111 on issue 164 by [email protected]: Wrong order in Object properties interation
http://code.google.com/p/v8/issues/detail?id=164
There seems to be a widespread feeling that this used to work the way people expected it, but then the V8 team broke it in order to be mean.
What actually happened was that originally the order was completely arbitrary in V8. At a later point it was changed so that non-numeric indices were in insertion order, and numeric indices were sometimes in insertion order. Whether or not the numeric indices were in in insertion order was dependent on internal V8 heuristics that decide whether to use an array or a hash map implementation for the numeric indices. Making heuristics in the V8 implementation visible in this way was felt to be undesirable so it was normalized so that numeric indices were always iterated in numeric order regardless of the internal representation. Numeric iteration order was always a possibility, but with the last change it was made predictable.
There has never been any difference between the internal representation or iteration order of arrays vs. other objects in V8.
Here is an independent test of the way arrays and objects perform in various engines (a little out of date now): http://news.qooxdoo.org/javascript-array-performance-oddities-characteristics If this bug ever gets 'fixed' you can wave goodbye to some of the nice performance results in that graph.
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